A hybrid electric vehicle (HEV) is understood to be a motor vehicle that is driven by a hybrid drive having at least one electrical machine and an internal combustion engine, such as an Otto or Diesel engine. It draws the energy from a storage device for electrical energy in the motor vehicle and from an operating fuel tank. A hybrid electric vehicle such as this is also known as a hybrid vehicle, hybrid car or vehicle with a hybrid drive.
The hybrid drive is employed to improve efficiency, to reduce the use of fossil fuels or to increase power in the low engine speed range. An electrical power supply to the vehicle is possible in the configuration as a plug-in hybrid.
In a parallel hybrid drive, there is an operating mode in which the electrical machine and the internal combustion engine act upon the drive train at the same time, which sums the torques of the individual drives. This permits a less powerful design for all engines, which reduces costs, weight and installation space, as well as fuel in the case of the internal combustion engine, by downsizing. Parallel hybrids can be implemented comparatively cost-effectively as mild hybrids. If a purely electrical driving mode should also be possible, the electric motor must be designed accordingly.
If the electrical machine is connected to the driving wheels of the hybrid electric vehicle in a force-transmitting manner, and if a manual transmission is arranged between the internal combustion engine and the driving wheels and a clutch is provided between the internal combustion engine and the manual transmission, then the internal combustion engine can be separated from the drive train by opening the clutch, whereupon only the electrical machine, operated as a motor, provides propulsion in the driving direction. Thus, the internal combustion engine does not have to be coupled during purely electrical operation. Here the electrical machine can be coupled without a clutch—i.e. rigidly—to the driving wheels, or the electrical machine can be coupled to the driving wheels via a clutch, a freewheel clutch or a shiftable transmission. The electrical machine can drive the motor vehicle while the internal combustion engine is not engaged, i.e. the clutch associated with the internal combustion engine is open.
However, depending upon the gear of the manual transmission that is engaged, the situation can arise that a closing of the clutch results in a speed increase in the internal combustion engine, which can damage the engine, or the closing of the clutch results in a speed decrease in the internal combustion engine to a value below a minimum operating speed that stalls the engine.